A case is reported of anaphylactoid response to thiopentone, used for the induction of anaesthesia for cystoscopy in a 56-year-old woman. The dose precipitating this reaction was only 50 mg and the patient had no previous history of any allergic tendency. A full recovery was made, though gastrointestinal disturbances persisted for several days. The treatment, differential diagnosis, investigations and recommendations for subsequent anaesthetic management are discussed.